Beacon marker interrupt device

ABSTRACT

Apparatus for providing adjustable time-limited attenuation of a marker beacon receiver audio signal, comprising a variable attentuator connected between the audio signal source and the audio signal output, and shunted by a normally closed timer switch. An adjustable RC timing circuit, actuated by an automatic or manually operable mute switch, drives a two state amplifier, the output of which is connected to and operates the timer switch.

United States Patent [1 1 Messinger [451 July 24, 1973 [54] BEACONMARKER INTERRUPT DEVICE 2,174,641 10/1939 Sininger 325/478 3,621,28411/1971 Cluett et al. 307/237 [75] Invent $3521";- Mess'nle" Center3,665,320 5 1972 Ohsawa et al. 325/478 [73] Assignee: Narco ScientificIndustries, Inc., Ft. Primary Examiner-Stanley D. Miller, Jr.

Washington, Pa. Attorney-Henry N. Paul, Jr.. Albert W. Preston. 22Filed: Dec. 6, 1971 [21] Appl. No.: 205,088 57 ABSTRACT [52] U S Cl307/237 179/1 SW 179/1 VL Apparatus for providing adjustabletime-limited attenu' 325/478 328/l68 330/51 ation of a marker beaconreceiver audio signal, com- 1 Int Cl nosk 5/08 prising a variableattentuator connected between the Fie'ld SW 1 vb audio signal source andthe audio signal output, and 325/478 330/51 328/168 shunted by anormally closed timer switch. An adjust- 172 able RC timing circuit,actuated by an automatic or manually operable mute switch, drives a twostate amplifier, the output of which is connected to and oper- [56]References cued ates the timer switch;

UNITED STATES PATENTS 2,520,732 8/1950 McDonald, Jr 179/1 VL X 7 Claims,3 Drawing Figures 23 SlGNAL s l e r fit v- ATTENUATOR OUTPUT DETECT PEAKVOLTAGE A+ TIMER INPUT REGl/JLATOR BEACON MARKER INTERRUPT DEVICEBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionlies in the field of audio muting devices and, more particularly,devices for providing adjustable time period muting of an aircraftmarker beacon receiver audio output.

2. Description of the Prior Art In an airport instrument landing system,commonly referred to as ILS, a pair of marker beacons are provided inline with the runway, the purpose of which beacons is to provideinformation to the pilot concerning the ground distance of the planefrom the runway. The marker beacons consist of relatively thin fanshapedradio beams directed generally vertically from the ground, with eachbeacon radiating approximately 2 watts at 75 megahertz. The outermarker, generally located 4 to 7 miles from the end of the runway,carries a 400 Hz modulation which is received and detected as an audiosignal, generally two dashes per second. A middle marker is placed about3,500 feet from the end of the runway, and is modulated at 1,300 Hz inthe form of a dot and dash signal. Thus, as the pilot brings hisaircraft in toward the runway along the localizer path, he firstreceives an audio signal from the outer marker beacon, which audiosignal is maintained as long as the plane is passing through the patternof the outer marker beacon. After passing through such outer markerbeacon, at some subsequent point the plane enters the middle markerbeacon, where the middle marker audio signal is detected and maintainedwhile the plane passes through such middle beacon.

In practice, the aircraft receiver which produces the audio signal iswithout any volume control circuitry, such that as the plane enters amarker beacon pattern, the marker audio signal increases in intensityuntil the aircraft is approximately directly over the beacontransmitter, and then decreases until the plane is entirely out of thebeacon pattern. For typical approach speeds, the beacon signal may bepicked up 15 to seconds before the audio peak, and will be maintainedfor a comparable amount of time after such peak. As is well known bythose familiar with ILS flight operation, the beacon audio signal may bevery bothersome, particularly when noted that for at least half of itsduration it is not providing any information, since the aircraftoperator has already noted his position. Consequently, there is a greatdesire among aircraft operators for the capability of attenuating thebeacon audio signal after the pilot detects the audio peak. However, asimple audio-attenuator device, whereby the pilot would limit the audiovolume, would not be feasible, since in many instances the pilot wouldnot remember to turn the audio signal back on and would consequentlymiss the signal provided by the middle beacon. There thus exists a needfor an efficient and reliable device for obtaining a timed attenuationof the marker beacon audio signal, such that the pilot can attenuate thesignal when he so desires, and yet have the beacon circuit ready toprovide the audio signal upon passage through the next marker.

Another application where there is a need for a marker beacon interruptdevice is where a pilot is flying his plane over an airport, but withoutintention of entering the flight pattern at such airport. This commonlyoccurs during cross country flights, and particularly where VOR stationsare located at or near an airport." In such situations, the pilotdesires to attenuate the beacon audio signal while in the vicinity ofthe airport.

However, here again, if the pilot simply turned off the beacon audiovolume, he quite likely would forget to turn it back after leaving therange of the beacon, and could well miss it when it was needed at alater time during the flight.

BRIEF DESCRIPTION OF THE DRAWINGS SUMMARY OF THE INVENTION It is aprimary object of this invention to provide a device for interruptingthe marker beacon receiver audio signal in an airplane for an adjustableduration of time.

It is a further object of this invention to provide a device forattenuating the marker beacon audio signal received in an airplane for aduration of time, after which duration of time the marker beacon audiosignal is received in the normal manner. The duration may be internallyadjusted in accordance with the aircraft approach speed.

In accordance with the above objectives, there is provided an adjustableattenuator which is placed in series with the marker beacon audiooutput, which attenuator is shunted by a normal closed electronic gate,which gate is switchably operated by a two-state operational amplifierwith an RC timing circuit connected to one of its inputs. Operation of amute switch connected to the RC circuit holds the amplifier in a secondstate for appreciably the discharge time of the RC circuit, thus holdingthe gate in an open position, and placing the variable attenuator in thepath of the marker beacon audio signal. Upon discharge of the timingcircuit, the amplifier returns to its flrst state, and the attenuator isagain shunted by the low impedance closed gate.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I, theoperation of the device of this invention is shown in block diagramform. The audio signal input, appearing at terminal 22, is connected tooutput terminal 23 through attenuator 21. The means of deriving theaudio signal in the receiver, and transmitting it to terminal 22 withsufficient power, is well known in the art, and does not form a part ofthis invention. Attenuator 21 comprises a normally closed electricallyactuated gate, or switch, having a gate input terminal which isconnected to a timer 25. The normal output from timer 25 allows theswitch to exist in its closed position. Timer 25 is powered by a voltageregulator 26, and is set in motion by .momentary manual or automaticoperation of mute switch 27. When switch 27 is closed, timer 25 producesa timed output signal which causes the switch to open, thereby placingan attenuator impedance directly in series between terminals 22 and 23,thus providing the desired attenuation. At the end of the preset timeperiod, the output signal from timer 25 reverts to its prior state, suchthat the attenuating impedance is again shunted, in which stateattenuator 21 introduces minimal insertion loss.

Referring now to FIG. 2, attenuator 21 is shown to be preferablycomprised of a variable resistance 40 (suitably a manually adjustablepotentiometer) in parallel with an enhancement mode FET 41. FET 41 hasits output terminals tied to terminals 22 and 23 respectively, and is inan on" condition, and provides a substantially closed switch (very lowimpedance) shunting resistor 40 when its gate is open or is driven by alow voltage signal. Consequently, under normal conditions resistor 40provides no attenuation of the audio signal, with only minimal insertionloss due to the presence of PET 41 in the circuit.

DC power, designated A+, is connected to terminal 28 from any externalvoltage source. The A+ voltage is regulated through resistor 29 andzener diode 30, to provide a regulated DC voltage of about volts betweenterminal 44 and ground. With this regulator, the line voltage suppliedat terminal 28 may drop as low as l0 volts, and in practice may be ashigh as about 32 volts. The regulated voltage is supplied through twopaths to the two input terminals 38-1 and 38-2 of comparator-amplifierunit 38. Unit 38 is a bi-stable lC device, such as is commerciallyavailable from National Semiconductor Corporation, Motorola, orFairchild Camera. It has a characteristic of providing a low output(about 2 volts) when the voltage on terminal 38-2 is plus with respectto the voltage on terminal 38-1, and a high output (about 9 volts) whenthe voltage on terminal 38-2 is below, or less plus than that onterminal 38-1. The voltage on terminal 38-1 is provided by the voltagedivider comprising resistors 36 and 37 connected between terminal 44 andground, which divider provides a fixed fraction of the regulatedvoltage. The voltage on terminal 38-2 is provided by the RC circuitcomprising capacitor 31 and the parallel combination of resistors 32 and33 in series with current limiting resistor 34.

Capacitor 31 is connected between terminal 44, carrying the regulatedvoltage, and input terminal 38-2. Connected across capacitor 31 is aresistive path consisting of resistor 32 and variable resistor 33 inparallel, which parallel combination is in turn connected in series withresistor 34. Terminal 38-2 is also connected through muting switch 27 toground. Switch 27 may be any manual or automatic momentary closed typeswitch which, when closed, provides a sufficiently low conductance pathbetween terminal 38-2 and ground to permit capacitor 31 to substantiallydischarge. For example, as illustrated at FIG. 3, switch 27 may consistof a transistor switch, as shown at 27-(), which is turned on for ashort interval by a signal derived from the marker beacon audio,suitably processed, and connected to transistor input terminal 27-N.Suitably, the approximate peak of the audio signal is detected,corresponding to the point where the aircraft is substantially over thetransmitter of the marker beacon and then transistor switch 27-0 isgated on.

In operation, with switch 27 open, the voltage on terminal 38-2 isappreciably the regulated voltage derived across the zener diode, and ismore positive than the divided voltage on terminal 384. Consequently,the output of unit 38 is low, holding the switch 41 in a closedcondition. When switch 27 is closed, terminal 38-2 is driven to ground,such that it is less positive than terminal 38-1, causing a high outputsignal from unit 38, and causing switch 41 to open and place theattenuator resistor 40 in the audio circuit. During the brief momentthat switch 27 is closed, capacitor 31 quickly charges to appreciablythe regulated voltage,

5 and after switch 27 is again opened, the capacitor 31 dischargesthrough resistors 32, 33, and 34. Resistor 32 is suitably 500 K, andresistor 33 is suitably a 500 K potentiometer. Resistor 34 is a lowvalue (suitably 2.2 K) current limiting resistor, to guard againstexcess currents in case resistor 33 is made close to a short circuit andswitch 27 is closed. Thus, the time constant of the timing circuit isappreciably determined by capacitor 31 and the parallel combination ofresistors 32 and 33. By varying resistor 33, the time constant can, ofcourse, be varied. As soon as capacitor 31 has discharged to a pointwhere the voltage on terminal 38-2 becomes equal to or greater than thaton terminal 38-1, the output of unit 38 reverts to its low state, FET 41becomes essentially a closed switch, and attenuator resistor 40 iseffectively taken out of the circuit. Thus, the time period forattenuation, during which resistor 40 attenuates the audio signal, isdetermined both by the RC constant and the voltage divider 36, 37, sinceunit 38 switches when the compared signals on its two inputs reverse inrelative magnitude.

From the above, it is seen that the apparatus of this invention providesa simple, efficient and reliable circuit for providing an aircraft withmeans for safe interruption of a marker beacon audio signal. The amountof audio attenuation provided during the timing cycle is adjustable byvarying resistor 40. It has been found that by using a 500 K pot forresistor 40, audio attenuation can be adjusted between 2 db and 60 db,assuming a 600 ohm source and load impedance. FET 41 provides a minimalinsertion loss of 2 db for a 600 ohm system. Further, the period ofaudio interrupt may be conveniently varied by adjusting pot 33, toprovide the correct interrupt period according to approach speed and-/or other factors. For applications where a timed interrupt might not bedesirable, the marker beacon audio signal may be processed to firstdetect the peak, and then detect when the audio level has dropped backdown to a pre-determined level. When the lower level is reached, are-set signal is derived to re-set the attenuator switch.

The apparatus of this invention may be easily installed in an aircraftwithout disruption of the existing navigation electrical system. Boththe audio input and output terminals may be maintained at groundreference. If the line voltage,A+, is removed or lost, the circuitreverts to the minimum attenuation state, due to the normal closedcondition of PET 41. Muting control is effected by a single wire whichis switched to ground, and operation of the muting switch does notintroduce any pops, cracks or other transients into the audio signal.

I claim:

1. Apparatus for timed interruption of a marker beacon audio signal,comprising:

a. attenuator means, connected in the path of said audio signal, andhaving switchable attenuating and non-attenuating states and an inputfor receiving a switching signal;

5 b. switching signal means, for switching said attenuator means intoone of said states, having an output connected to said input of saidattenuator means, and having first and second input terminals, the

output of said switching signal means switching said attenuator means toa first low attenuation state when the signal connected to said firstinput terminal is greater than the signal connected to said second inputterminal, and driving said attenuator means to a second high attenuationstate when the signal connected to the second input terminal is greaterthan the-signal connected to the first input terminal,

0. timing means, connected to said first and second inputs, and adaptedto provide a time varying signal to one of said inputs, and a constantsignal to the other of said inputs said constant signal being such thatsaid switching means normally maintains said attenuator means in its lowattenuation state, and

d. switch means, connected to said timing means for triggering saidtiming means through a timing cycle, whereby said switching circuitmeans switches said attenuator means into said attenuating state,causing attenuation of said audio signal during such timing cycle.

2. The apparatus as described in' claim 1, wherein said attenuator meanscomprises a variable resistor shunted by a semiconductor switch.

3. The apparatus as described in claim 2, wherein said semiconductorswitch is an enhancement mode FET.

4. The apparatus as described in claim 1, wherein said switching signalmeans comprises a comparator amplifier.

5. The apparatus as described in claim 1, wherein said timing meanscomprises an RC circuit, with a variable resistor.

6. The apparatus as described in claim 1, wherein said timing meansincludes a divider circuit for determining the relative amplitudes ofthe signals connected to said first and second input terminalsrespectively.

7. A method for providing a timed interruption of a detected markerbeacon audio signal in an aircraft, comprising:

a. detecting the approximate peak of said audio signal, corresponding tothe point where said aircraft is substantially over the transmitter ofsaid marker beacon;

b. attenuating said audio signal by switchably placing an attenuator inthe electrical path of said detected audio signal;

c. automatically maintaining said attenuator in said path for apredetermined time; and

d. automatically removing said attenuator after said predeterminedperiod is timed.

1. Apparatus for timed interruption of a marker beacon audio signal,comprising: a. attenuator means, connected in the path of said audiosignal, and having switchable attenuating and non-attenuating states andan input for receiving a switching signal; b. switching signal means,for switching said attenuator means into one of said states, having anoutput connected to said input of said attenuator means, and havingfirst and second input terminals, the output of said switching signalmeans switching said attenuator means to a first low attenuation statewhen the signal connected to said first input terminal is greater thanthe signal connected to said second input terminal, and driving saidattenuator means to a second high attenuation state when the signalconnected to the second input terminal is greater than the signalconnected to the first input terminal, c. timing means, connected tosaid first and second inputs, and adapted to provide a time varyingsignal to one of said inputs, and a constant signal to the other of saidinputs said constant signal being such that said switching meansnormally maintains said attenuator means in its low attenuation state,and d. switch means, connected to said timing means for triggering saidtiming means through a timing cycle, whereby said switching circuitmeans switches said attenuator means into said attenuating state,causing attenuation of said audio signal during such timing cycle. 2.The apparatus as described in claim 1, wherein said attenuaTor meanscomprises a variable resistor shunted by a semiconductor switch.
 3. Theapparatus as described in claim 2, wherein said semiconductor switch isan enhancement mode FET.
 4. The apparatus as described in claim 1,wherein said switching signal means comprises a comparator amplifier. 5.The apparatus as described in claim 1, wherein said timing meanscomprises an RC circuit, with a variable resistor.
 6. The apparatus asdescribed in claim 1, wherein said timing means includes a dividercircuit for determining the relative amplitudes of the signals connectedto said first and second input terminals respectively.
 7. A method forproviding a timed interruption of a detected marker beacon audio signalin an aircraft, comprising: a. detecting the approximate peak of saidaudio signal, corresponding to the point where said aircraft issubstantially over the transmitter of said marker beacon; b. attenuatingsaid audio signal by switchably placing an attenuator in the electricalpath of said detected audio signal; c. automatically maintaining saidattenuator in said path for a predetermined time; and d. automaticallyremoving said attenuator after said predetermined period is timed.